Demonstration of Imino Acids as Products of the Reactions Catalyzed by D- and L-Amino Acid Oxidases

It had long been thought, but never demonstrated, that imino acids are formed in the reactions catalyzed by D- and L-amino acid oxidases (EC 1.4.3.3 and 1.4.3.2). The formation of imino acids is now shown directly by allowing the amino acid oxidase reaction to proceed in the presence of NaBH4, when the imino acid is reduced to the corresponding racemic amino acid. Thus, when NaBH4is added to a mixture of D-amino acid oxidase and D-alanine, a significant amount of L-alanine is formed. Analogous results are obtained using L-amino acid oxidase and L-leucine. Since D-amino acid oxidase is active in the presence of NaBH4, L-alanine continues to be formed until most of the D-isomer is oxidized by the enzyme. This reaction provides a new method for inverting the configuration of an amino acid. When NaBH4is added to a system containing D-amino acid oxidase plus D-alanine and L-lysine, free ε -N-(1-carboxyethyl)-L-lysine is formed. When bovine serum albumin is substituted for L-lysine, the same compound results upon acid hydrolysis. It is concluded that the amino acid oxidase reaction produces a free imino acid, which may be reduced by NaBH4to a racemic amino acid or may form Schiff's bases by reaction with the ε -amino groups of proteins and of free lysine.